Process for fabricating circuit assemblies using electrodepositable dielectric coating compositions

ABSTRACT

Provided is a process for forming metallized vias in a substrate including the steps of (I) applying to an electroconductive substrate an electrodepositable coating composition onto all exposed surfaces of the substrate to form a conformal dielectric coating; (II) ablating a surface of the dielectric coating to expose a section of the substrate; (III) applying a layer of metal to all surfaces to form metallized vias in the substrate. Also disclosed are processes for fabricating a circuit assembly which include the application of an electrodoepositable coating composition onto exposed surfaces of the substrate/core to form a conformal dielectric coating thereon. The electrodepositable coating composition includes a resinous phase dispersed in an aqueous phase, where the resinous phase has a covalently bonded halogen content of at least 1 percent by weight. The dielectric coating derived therefrom has a low dielectric constant and low dielectric loss factor.

FIELD OF THE INVENTION

[0001] The present invention relates to processes for forming metallizedvias and for fabricating multi-layer circuit assemblies comprising adielectric coating, particularly a dielectric coating applied byelectrodeposition.

BACKGROUND OF THE INVENTION

[0002] Electrical components, for example, resistors, transistors, andcapacitors, are commonly mounted on circuit panel structures such asprinted circuit boards. Circuit panels ordinarily include a generallyflat sheet of dielectric material with electrical conductors disposed ona major, flat surface of the sheet, or on both major surfaces. Theconductors are commonly formed from metallic materials such as copperand serve to interconnect the electrical components mounted to theboard. Where the conductors are disposed on both major surfaces of thepanel, the panel may have via conductors extending through holes (or“through vias”) in the dielectric layer so as to interconnect theconductors on opposite surfaces. Multi-layer circuit panel assemblieshave been made heretofore which incorporate multiple stacked circuitpanels with additional layers of dielectric materials separating theconductors on mutually facing surfaces of adjacent panels in the stack.These multi-layer assemblies ordinarily incorporate interconnectionsextending between the conductors on the various circuit panels in thestack as necessary to provide the required electrical interconnections.

[0003] In microelectronic circuit packages, circuits and units areprepared in packaging levels of increasing scale. Generally, thesmallest scale packaging levels are typically semiconductor chipshousing multiple microcircuits and/or other components. Such chips areusually made from ceramics, silicon, and the like. Intermediate packagelevels (i.e., “chip carriers”) comprising multi-layer substrates mayhave attached thereto a plurality of small-scale chips housing manymicroelectronic circuits. Likewise, these intermediate package levelsthemselves can be attached to larger scale circuit cards, motherboards,and the like. The intermediate package levels serve several purposes inthe overall circuit assembly including structural support, transitionalintegration of the smaller scale microcircuits and circuits to largerscale boards, and the dissipation of heat from the circuit assembly.Substrates used in conventional intermediate package levels haveincluded a variety of materials, for example, ceramic, fiberglassreinforced polyepoxides, and polyimides.

[0004] The aforementioned substrates, while offering sufficient rigidityto provide structural support to the circuit assembly, typically havethermal coefficients of expansion much different than that of themicroelectronic chips being attached thereto. As a result, failure ofthe circuit assembly after repeated use is a risk due to failure ofadhesive joints between the layers of the assembly.

[0005] Likewise, dielectric materials used on the substrates must meetseveral requirements, including conformality, flame resistance, andcompatible thermal expansion properties. Conventional dielectricmaterials include, for example, polyimides, polyepoxides, phenolics, andfluorocarbons. These polymeric dielectrics typically have thermalcoefficients of expansion much higher than that of the adjacent layers.

[0006] There has been an increasing need for circuit panel structureswhich provide high density, complex interconnections. Such a need can beaddressed by multi-layer circuit panel structures, however, thefabrication of such multi-layer circuit assemblies has presented seriousdrawbacks.

[0007] Generally multi-layer panels are made by providing individual,dual sided circuit panels with appropriate conductors thereon. Thepanels are then laminated one atop the other with one or more layers ofuncured or partially cured dielectric material, commonly referred to as“prepregs” disposed between each pair of adjacent panels. Such a stackordinarily is cured under heat and pressure to form a unitary mass.After curing, holes typically are drilled through the stack at locationswhere electrical connections between different boards are desired. Theresulting holes or “through vias” are then coated or filled withelectrically conductive materials usually by plating the interiors ofthe holes to form a plated through via. It is difficult to drill holeswith a high ratio of depth to diameter, thus the holes used in suchassemblies must be relatively large and consume a great deal of space inthe assembly.

[0008] U.S. Pat. No. 6,266,874 B1 discloses of method of making amicroelectronic component by providing a conductive substrate or “core”;providing a resist at selected locations on the conductive core; andelectrophoretically depositing an uncured dielectric material on theconductive core except at locations covered by the resist. The referencesuggests that the electrophoretically deposited material can be acationic acrylic- or cationic epoxy-based composition as those known inthe art and commercially available. The electrophoretically depositedmaterial then is cured to form a conformal dielectric layer, and theresist is removed so that the dielectric layer has openings extending tothe conductive core at locations which had been covered by the resist.The holes thus formed and extending to the coated substrate or “core”are commonly referred to as “blind vias”. In one embodiment, thestructural conductive element is a metal sheet containing continuousthrough holes or “through vias” extending from one major surface to theopposite major surface. When the dielectric material is appliedelectrophoretically, the dielectric material is deposited at a uniformthickness onto the conductive element surface and the hole walls. It hasbeen found, however, that the electrophoretically deposited dielectricmaterials suggested by this reference can be flammable, and thus do notmeet typical flame retardancy requirements.

[0009] U.S. Pat. Nos. 5,224,265 and 5,232,548 disclose methods offabricating multi-layer thin-film wiring structures for use in circuitassemblies. The dielectric applied to the core substrate preferably is afully cured and annealed thermoplastic polymer such aspolytetrafluoroethylene, polysulfone, or polyimide-siloxane, preferablyapplied by lamination.

[0010] U.S. Pat. No. 5,153,986 discloses a method of fabricating metalcore layers for a multi-layer circuit board. Suitable dielectricsinclude vapor-depositable conformal polymeric coatings. The method usesperforate solid metal cores and the reference describes generallycircuitization of the substrate.

[0011] U.S. Pat. No. 4,601,916 suggests that while electrodeposition ofan insulating coating directly to the metal wall portions of the holescan create a uniform film of resin on the hole walls without producingthinning of the coating at the top and bottom rims of the holes, thesubsequent metal deposits would not adhere to the hole walls and,further, that the electrical insulating properties were inadequate.Hence, the reference is directed to an improved method for formingplated through holes in metal core printed circuit boards byelectrophoretically depositing coatings thereon which comprise anelectrodepositable resinous coating including a solid inorganic fillerin finely divided form. Suitable fillers include clays, silica, alumina,silicates, earths and the like. The composition exhibits a volumeresistivity greater than 10⁴ megohm-cm between the printed circuitconductor and the metal core. The method comprises electrophoreticallydepositing the aforementioned composition onto the metal wall portionsof the holes; curing the resinous coating, the thickness of which beingat least 0.025 millimeters; creating a hydrophilic microetched surfaceon the coating with an aqueous oxidizing solution to promote adhesion;depositing a metal layer on the surface of the resinous coating on thehole walls and on the insulating surface layers, the metal layeradhering to the coating with a specified peel strength, and forming aprinted circuit on the insulated metal substrate by standard printedcircuit techniques.

[0012] U.S. Pat. No. 4,238,385 discloses coating compositions forelectrophoretic application to electroconductive substrates for printedcircuits. The compositions comprise a pigment-containing finely dividedsynthetic resin powder where the resin includes an epoxy resin and thepigment includes 2 to 10 weight parts of a finely-divided silica,admixed with a cationic resin. The composition forms an insulating filmon the electroconductive substrate which is suitable for printedcircuits providing desirable properties such as dimensional stabilityand mechanical strength.

[0013] Circuitization of intermediate package levels is conventionallyperformed by applying a positive- or negative-acting photoresist(hereinafter collectively referred to as “resist”) to the metallizedsubstrate, followed by exposure, development, etching and stripping toyield a desired circuit pattern. Resist compositions typically areapplied by laminating, spray, or immersion techniques. The resist layerthus applied can have a thickness of 5 to 50 microns.

[0014] In addition to the substrates previously mentioned, conventionalsubstrates for intermediate package levels can further include solidmetal sheets such as those disclosed in U.S. Pat. No. 5,153,986. Thesesolid structures must be perforated during fabrication of the circuitassembly to provide through vias for alignment purposes.

[0015] In view of the prior art processes, there remains a need in theart for multilayer circuit panel structures which provide high densityand complex interconnections, the fabrication of which overcomes thedrawbacks of the prior art circuit assemblies.

SUMMARY OF THE INVENTION

[0016] In one embodiment, the present invention is directed to a processfor forming metallized vias in a substrate. The process comprises thesteps of: (I) electrophoretically applying to an electroconductivesubstrate an electrodepositable coating composition onto all exposedsurfaces of the substrate to form a conformal dielectric coatingthereon, the electrodepositable coating composition comprising aresinous phase dispersed in an aqueous phase, the resinous phasecomprising: (a) an ungelled, active hydrogen-containing, ionic groupcontaining resin, and (b) a curing agent reactive with the activehydrogens of the resin (a), the resinous phase having a covalentlybonded halogen content of at least 1 percent by weight based on totalweight of resin solids present in the resinous phase; (II) ablating asurface of the conformal dielectric coating in a predetermined patternto expose a section of the substrate; and (III) applying a layer ofmetal to all surfaces to form metallized vias in the substrate.

[0017] In another embodiment, the present invention relates to a processfor fabricating a circuit assembly comprising the steps of: (I)providing an electroconductive core; (II) applying electrophoreticallythe electrodoepositable coating composition described above onto allexposed surfaces of the core to form a conformal dielectric coatingthereon; (III) ablating a surface of the conformal dielectric coating ina predetermined pattern to expose a section of the core; (IV) applying alayer of metal to all surfaces to form metallized vias in the core; and(V) applying a resinous photosensitive layer to the metal layer.

[0018] The present invention also is directed to a process forfabricating a circuit assembly comprising the steps of: (I) providing anelectroconductive core; (II) providing a photoresist at predeterminedlocations on the surface of the core; (III) applying electrophoreticallyan electrodepositable coating composition described above over the coreof step (II), wherein the coating composition is depositedelectrophoretically over all surfaces of the core except at thelocations having the photoresist thereon; (IV) curing theelectrophoretically applied coating composition to form a curedconformal dielectric layer over all surfaces of the core except at thelocations having the photoresist thereon; (V) removing the photoresistto form a circuit assembly having vias extending to the core at thelocations previously covered with the resist; and (VI) optionally,applying a layer of metal to all surfaces of the circuit assembly ofstep (V) to form metallized vias extending to the core.

[0019] The present invention is further directed to a substrate andcircuit assemblies coated by the respective aforementioned processes.

DETAILED DESCRIPTION OF THE INVENTION

[0020] Other than in the operating examples, or where otherwiseindicated, all numbers expressing quantities of ingredients, reactionconditions and so forth used in the specification and claims are to beunderstood as being modified in all instances by the term “about.”Accordingly, unless indicated to the contrary, the numerical parametersset forth in the following specification and attached claims areapproximations that may vary depending upon the desired propertiessought to be obtained by the present invention. At the very least, andnot as an attempt to limit the application of the doctrine ofequivalents to the scope of the claims, each numerical parameter shouldat least be construed in light of the number of reported significantdigits and by applying ordinary rounding techniques.

[0021] Notwithstanding that the numerical ranges and parameters settingforth the broad scope of the invention are approximations, the numericalvalues set forth in the specific examples are reported as precisely aspossible. Any numerical values, however, inherently contain certainerrors necessarily resulting from the standard deviation found in theirrespective testing measurements.

[0022] Also, it should be understood that any numerical range recitedherein is intended to include all sub-ranges subsumed therein. Forexample, a range of “1 to 10” is intended to include all sub-rangesbetween and including the recited minimum value of 1 and the recitedmaximum value of 10, that is, having a minimum value equal to or greaterthan 1 and a maximum value of equal to or less than 10.

[0023] As previously mentioned, in one embodiment, the present inventionis directed to a process for forming metallized vias in a substrate, theprocess comprising: (I) electrophoretically applying to anelectroconductive substrate an electrodepositable coating composition(described in detail below) onto all surfaces of the substrate to form aconformal dielectric coating thereon; (II) ablating a surface of thedielectric coating in a predetermined pattern to expose a section of thesubstrate; and (III) applying a layer of metal to all surfaces of thesubstrate of step (II) to form metallized vias in the substrate.Optionally, the process further includes step (IV) applying aphotosensitive layer, as described below, to the metal layer.

[0024] In further embodiments, the present invention is directed toprocesses for fabricating multi-layer circuit assemblies. In oneembodiment, the present invention is directed to a process forfabricating a circuit assembly comprising the steps of: (I) providing anelectroconductive core, typically a metal core as discussed below; (II)applying electrophoretically any of the previously discussedelectrodepositable coating compositions onto all exposed surfaces of thecore to form a conformal dielectric coating thereon; (III) ablating asurface of the conformal dielectric coating in a predetermined patternto expose a section of the core; (IV) applying a layer of metal forexample, copper, to all surfaces to form metallized vias in the core;and (V) applying a resinous photosensitive layer to the metal layer.

[0025] The substrate or core can comprise any of a variety ofelectroconductive substrates, particularly metal substrates, forexample, untreated or galvanized steel, aluminum, gold, nickel, copper,magnesium or alloys of any of the foregoing metals, as well asconductive carbon coated materials. Also, the core has two majorsurfaces and edges and can have a thickness ranging from 10 to 100microns, typically from 25 to 100 microns.

[0026] It should be understood that for purposes of the processes of thepresent invention the formation of metallized vias “in the core” isintended to encompass the formation of “through vias” (i.e., theformation of metallized holes extending through the core from one majorsurface to the other) to provide through connections, as well as theformation of “blind vias” (i.e., the formation of metallized holesextending through the dielectric coating only to, but not through, thecore) to provide connections, for example, to ground or power. Also, forpurposes of the present invention, the formation of metallized viasextending “through the core” is intended to encompass the formation ofthrough vias only. Likewise, the formation of metallized vias extending“to the core” is intended to encompass the formation of blind vias only.

[0027] In a particular embodiment of the present invention, the core isa metal substrate selected from perforate copper foil, an iron-nickelalloy or combinations thereof. In one embodiment of the presentinvention, the core comprises an iron-nickel alloy commerciallyavailable as INVAR (trademark of Imphy S. A., 168 Rue de Rivoli, Paris,France) comprising approximately 64 weight percent iron and 36 weightpercent nickel. This alloy has a low coefficient of thermal expansioncomparable to that of the silicon materials typically used to preparechips. This property is desirable in order to prevent failure ofadhesive joints between successively larger or smaller scale layers of achip scale package due to thermal cycling during normal use. When aniron-nickel alloy is used as the metal core, a layer of metal, usuallycopper, typically is applied to all surfaces of the iron-nickel alloycore to ensure optimal conductivity. This layer of metal as well as thatapplied in step (IV) can be applied by conventional means and forexample, by electroplating and metal vapor deposition techniques,electroless plating, and typically has a thickness of from 1 to 10microns.

[0028] By “perforate metal core” is meant a mesh sheet having aplurality of holes or vias spaced at regular intervals. The diameter ofthe holes usually is about 200 microns, but may be larger or smaller asnecessary, provided that the diameter is large enough to accommodate allthe layers applied in the process of the present invention without theholes becoming obstructed. The center-to-center spacing of the holestypically is about 500 microns, but, likewise, may be larger or smalleras necessary. Via density can range from 500 to 10,000 holes per squareinch (77.5 to 1,550 holes per square centimeter).

[0029] Any of the electrodepositable coating compositions described indetail below can be electrophoretically applied to the electroconductivecore. The applied voltage for electrodeposition may be varied and canbe, for example, as low as 1 volt to as high as several thousand volts,but typically between 50 and 500 volts. The current density is usuallybetween 0.5 ampere and 5 amperes per square foot (0.5 to 5 milliamperesper square centimeter) and tends to decrease during electrodepositionindicating the formation of an insulating conformal film on all exposedsurfaces of the core. As used herein, in the specification and in theclaims, by “conformal” film or coating is meant a film or coating havinga substantially uniform thickness which conforms to the substratetopography, including the surfaces within (but, preferably, notoccluding) the holes. After the coating has been applied byelectrodeposition, it typically is thermally cured at elevatedtemperatures ranging from 90° to 300° C. for a period of 1 to 40 minutesto form a conformal dielectric coating over all exposed surfaces of thecore.

[0030] It should be understood, that for purposes of the processes ofthe present invention, any of the electrodepositable coatingcompositions (described in detail below) can be applied by a variety ofapplication techniques other than electrodeposition which are well-knowin the art, for example, by roll-coating or spray applicationtechniques. In such instances, it may be desirable to prepare thecomposition at higher resin solids content. Also, for such applications,the resinous binder may or may not include solubilizing or neutralizingacids and amines to form cationic and anionic salt groups, respectively.

[0031] The dielectric coating is of uniform thickness and often can beno more than 50 microns, usually no more than 25 microns, and typicallyno more than 20 microns. A lower film thickness is desirable for avariety of reasons. For example, a dielectric coating having a low filmthickness allows for smaller scale circuitry. Also, a coating have a lowdielectric constant (as discussed above) allows for a dielectric coatinghaving lower film thickness and also minimizes capacitive couplingbetween adjacent signal traces.

[0032] Those skilled in the art would recognize that prior to theelectrophoretic application of the dielectric coating, the core surfacemay be pretreated or otherwise prepared for the application of thedielectric. For example, cleaning, rinsing, and/or treatment with anadhesion promoter prior to application of the dielectric may beappropriate.

[0033] After application of the dielectric coating, the surface of thedielectric coating is ablated in a predetermined pattern to expose oneor more sections of the core. Such ablation typically is performed usinga laser or by other conventional techniques, for example, mechanicaldrilling and chemical or plasma etching techniques.

[0034] Metallization is performed after the ablation step by applying alayer of metal to all surfaces, allowing for the formation of metallizedvias in the core. Suitable metals include copper or any metal or alloywith sufficient conductive properties. The metal is typically applied byelectroplating or any other suitable method known in the art to providea uniform metal layer. The thickness of this metal layer can range from1 to 50 microns, typically from 5 to 25 microns.

[0035] To enhance the adhesion of the metal layer to the dielectricpolymer, prior to the metallization step all surfaces can be treatedwith ion beam, electron beam, corona discharge or plasma bombardmentfollowed by application of an adhesion promoter layer to all surfaces.The adhesion promoter layer can range from 50 to 5000 Ångstroms thickand typically is a metal or metal oxide selected from chromium,titanium, nickel, cobalt, cesium, iron, aluminum, copper, gold, tungstenand zinc, and alloys and oxides thereof.

[0036] After metallization, a resinous photosensitive layer (i.e.“photoresist” or “resist”) can be applied to the metal layer.Optionally, prior to application of the photoresist, the metallizedsubstrate can be cleaned and/or pretreated; e.g., treated with an acidetchant to remove oxidized metal. The resinous photosensitive layer canbe a positive or negative photoresist. The photoresist layer can have athickness ranging from 1 to 50 microns, typically from 5 to 25 microns,and can be applied by any method known to those skilled in thephotolithographic processing art. Additive or subtractive processingmethods may be used to create the desired circuit patterns.

[0037] Suitable positive-acting photosensitive resins include any ofthose known to practitioners skilled in the art. Examples includedinitrobenzyl functional polymers such as those disclosed in U.S. Pat.No. 5,600,035, columns 3-15. Such resins have a high degree ofphotosensitivity. In one embodiment, the resinous photosensitive layeris a composition comprising a dinitrobenzyl functional polymer,typically applied by spraying.

[0038] In a separate embodiment, the resinous photosensitive layercomprises an electrodepositable composition comprising a dinitrobenzylfunctional polyurethane and an epoxy-amine polymer such as thatdescribed in Examples 3-6 of U.S. Pat. No. 5,600,035.

[0039] Negative-acting photoresists include liquid or dry-film typecompositions. Any of the previously described liquid compositions may beapplied by spray; roll-coating; spin-coating, curtain-coating,screen-coating, immersion coating, or electrodeposition applicationtechniques.

[0040] In one embodiment, the liquid photoresists are applied byelectrodeposition, usually by cationic electrodeposition.Electrodepositable photoresist compositions comprise an ionic, polymericmaterial which may be cationic or anionic, and may be selected frompolyesters, polyurethanes, acrylics, and polyepoxides. Examples ofphotoresists applied by anionic electrodeposition are shown in U.S. Pat.No. 3,738,835. Photoresists applied by cationic electrodeposition aredescribed in U.S. Pat. No. 4,592,816. Examples of dry-film photoresistsinclude those disclosed in U.S. Pat. Nos. 3,469,982, 4,378,264, and4,343,885. Dry-film photoresists are typically laminated onto thesurface such as by application of hot rollers.

[0041] Note that after application of the photosensitive layer, themulti-layer substrate may be packaged at this point allowing fortransport and processing of any subsequent steps at a remote location.

[0042] In an embodiment of the invention, after the photosensitive layeris applied, a photo-mask having a desired pattern may be placed over thephotosensitive layer and the layered substrate exposed to a sufficientlevel of a suitable radiation source, typically an actinic radiationsource. As used herein, the term “sufficient level of radiation” refersto that level of radiation which polymerizes the monomers in theradiation-exposed areas in the case of negative acting resists, or whichde-polymerizes the polymer or renders the polymer more soluble in thecase of positive acting resists. This results in a solubilitydifferential between the radiation-exposed and radiation-shielded areas.

[0043] The photo-mask may be removed after exposure to the radiationsource and the layered substrate developed using conventional developingsolutions to remove more soluble portions of the photosensitive layer,and uncover selected areas of the underlying metal layer. The metaluncovered may then be etched using metal etchants which convert themetal to water soluble metal complexes. The soluble complexes may beremoved by water spraying.

[0044] The photosensitive layer protects the underlying substrate duringthe etching step. The remaining photosensitive layer, which isimpervious to the etchants, may then be removed by a chemical strippingprocess to provide a circuit pattern connected by the metallized vias.

[0045] After preparation of the circuit pattern on the multi-layeredsubstrate, other circuit components may be attached to form a circuitassembly. Additional components include, for example, one or moresmaller scale components such as semiconductor chips, interposer layers,larger scale circuit cards or mother boards and active or passivecomponents. Note that interposers used in the preparation of the circuitassembly may be prepared using appropriate steps of the process of thepresent invention. Components may be attached using conventionaladhesives, surface mount techniques, wire bonding or flip chiptechniques. High via density in the multi-layer circuit assemblyprepared in accordance with the present invention allows for moreelectrical interconnects from highly functional chips to the packages inthe assembly.

[0046] In another embodiment, the present invention is directed to aprocess for fabricating a circuit assembly comprising the steps of (I)providing a core, such as any of the previously described metal cores;(II) providing a photoresist, such as any of the photoresistcompositions described above, at predetermined locations on the surfaceof the core; (III) applying electrophoretically any of theelectrodepositable coating compositions described in detail below overthe core of step (II), wherein the coating composition is depositedelectrophoretically over all surfaces of the core except at thelocations having the photoresist thereon; (IV) curing theelectrophoretically applied coating composition under the curingconditions described above to form a cured conformal dielectric layerover all surfaces of the core except at the locations having thephotoresist thereon; (V) removing the photoresist, as described above,to form a circuit assembly having vias extending to the metal core atthe locations previously covered with the resist; and (VI) optionally,applying a layer of metal, usually copper metal, to all surfaces of thecircuit assembly of step (V) by any of the previously described methodsfor metallizing to form metallized vias extending to the core. In aparticular embodiment of the present invention, prior to providing theresist in step (II) at predetermined locations on the surface of themetal core, a layer of metal, typically copper metal, is applied to themetal core.

[0047] The electrodepositable coating compositions suitable for use inany of the previously discussed processes comprise a resinous phasedispersed in an aqueous medium. The resinous phase comprises: (a) anungelled, active hydrogen-containing, ionic salt group-containing resin;and (b) a curing agent reactive with the active hydrogens of the resin(a).

[0048] In one embodiment of the present invention, the resinous phasehas a covalently bonded halogen content based on total weight of resinsolids present in said resinous phase such that when the composition iselectrodeposited and cured to form a cured film, the cured film passesflame resistance testing in accordance with IPC-TM-650, and has adielectric constant of less than or equal to 3.50. It should beunderstood that for purposes of the present invention, by “covalentlybonded halogen” is meant a halogen atom that is covalently bonded asopposed to a halogen ion, for example, a chloride ion in aqueoussolution.

[0049] The resinous phase of the electrodepositable coating compositionof the present invention can have a covalently bonded halogen content ofat least 1 weight percent, usually at least 2 weight percent, often atleast 5 weight percent, and typically at least 10 weight percent. Also,the resinous phase of the electrodepositable coating composition of thepresent invention can have a covalently bonded halogen content of lessthan 50 weight percent, usually less than 30 weight percent, often lessthan 25 weight percent, and typically less than 20 weight percent. Theresinous phase of the electrodepositable coating composition can have acovalently bonded halogen content which can range between anycombination of these values, inclusive of the recited values, providedthat the covalently bonded halogen content is sufficient to provide acured coating which passes flame resistance testing in accordance withIPC-TM-650 as described below.

[0050] Additionally, it should be noted that the covalently bondedhalogen content of the resinous phase can be derived from halogen atomscovalently bonded to one or both of the resin (a) and the curing agent(b), or halogen atoms covalently bonded to a compound (c) which isdifferent from and present as a component in the resinous phase of theelectrodepositable coating composition in addition to the resin (a) andthe curing agent (b).

[0051] As discussed above, for purposes of the present invention, flameresistance is tested in accordance with IPC-TM-650, Test Methods Manual,Number 2.3.10, “Flammability of Laminate”, Revision B, available fromthe Institute of Interconnecting and Packaging Electronic Circuits, 2215Sanders Road, Northbrook, Ill.

[0052] When the electrodepositable coating composition described aboveis electrophoretically deposited and cured to form a cured film (asdescribed in detail below), the cured film can have a dielectricconstant of no more than 3.50, often no more than 3.30, usually of nomore than 3.00, and typically no more than 2.80. Also, the cured filmtypically has a dielectric loss factor of less than or equal to 0.02,usually less than or equal to 0.15, and can be less than or equal to0.01.

[0053] A dielectric material is a non-conducting substance or insulator.The “dielectric constant” is an index or measure of the ability of adielectric material to store an electric charge. The dielectric constantis directly proportional to the capacitance of a material, which meansthat the capacitance is reduced if the dielectric constant of a materialis reduced. A low dielectric material is desired for high frequency,high speed digital where the capacitances of substrates and coatings arecritical to the reliable functioning of circuits. For example, presentcomputer operations are limited by coupling capacitance between circuitpaths and integrated circuits on multi-layer assemblies since computingspeed between integrated circuits is reduced by this capacitance and thepower required to operate is increased. See Thompson, Larry F., et al.,Polymers for Microelectronics, presented at the 203^(rd) NationalMeeting of American Chemical Society, Apr. 5-10, 1992.

[0054] The “dielectric loss factor” is the power dissipated by adielectric material as the friction of its molecules opposes themolecular motion produced by an alternating electric field. See I.Gilleo, Ken, Handbook of Flexible Circuits, at p. 242, Van NostrandReinhold, New York (1991). See also, James J. Licari and Laura A.Hughes, Handbook of Polymer Coatings for Electronics, pp. 114-18, 2^(nd)ed., Noyes Publication (1990) for a detailed discussion of dielectricmaterials and dielectric constant.

[0055] For purposes of the present invention, the dielectric constant ofthe cured electrodepositable coating composition is determined at afrequency of 1 megahertz using electrochemical impedance spectroscopy asfollows.

[0056] The coating sample is prepared by application of theelectrodepositable composition to a steel substrate and subsequentcuring to provide a cured dielectric coating having a film thickness of0.85 mil (20.83 microns). A 32 square centimeter free film of the cureddielectric coating is placed in the electrochemical cell with 150milliliters of electrolyte solution (1 M NaCl) and allowed toequilibrate for one hour. An AC potential of 100 mV is applied to thesample and the impedance is measured from 1.5 megahertz to 1 hertzfrequency range. The method employs a platinum-on-niobium expanded meshcounter electrode and a single junction silver/silver chloride referenceelectrode. The dielectric constant of the cured coating is determined bycalculating the capacitance at 1 megahertz, 1 kilohertz, and 63 hertz,and solving the following equation for E.

C=E _(o) EA/d

[0057] where C is the measured capacitance at discrete frequency (inFarads); E_(o) is the permitivity of free space (8.854187817¹²); A isthe sample area (32 square centimeters; d is the coating thickness; andE is the dielectric constant. It should be noted the values fordielectric constant as used in the specification and in the claims isthe dielectric constant determined as described above at a frequency of1 megahertz. Likewise, values for the dielectric loss factor as used inthe specification and in the claims is the difference between thedielectric constant measured at a frequency of 1 megahertz as describedabove, and the dielectric constant for the same material measured at afrequency of 1.1 megahertz.

[0058] The electrodepositable coating compositions useful in theprocesses of the present invention comprise as a main film-former, anungelled, active hydrogen ionic group-containing electrodepositableresin (a). A wide variety of electrodepositable film-forming polymersare known and can be used in the electrodepositable coating compositionsof the present invention so long as the polymers are “waterdispersible,” i.e., adapted to be solubilized, dispersed or emulsifiedin water. The water dispersible polymer is ionic in nature, that is, thepolymer can contain anionic functional groups to impart a negativecharge or cationic functional groups to impart a positive charge. In aparticular embodiment of the present invention, the resin (a) comprisescationic salt groups, usually cationic amine salt groups.

[0059] By “ungelled” is meant the resins are substantially free ofcrosslinking and have an intrinsic viscosity when dissolved in asuitable solvent, as determined, for example, in accordance withASTM-D1795 or ASTM-D4243. The intrinsic viscosity of the reactionproduct is an indication of its molecular weight. A gelled reactionproduct, on the other hand, since it is of essentially infinitely highmolecular weight, will have an intrinsic viscosity too high to measure.As used herein, a reaction product that is “substantially free ofcrosslinking” refers to a reaction product that has a weight averagemolecular weight (Mw), as determined by gel permeation chromatography,of less than 1,000,000.

[0060] Also, as used herein, the term “polymer” is meant to refer tooligomers and both homopolymers and copolymers. Unless stated otherwise,molecular weights are number average molecular weights for polymericmaterials indicated as “Mn” and obtained by gel permeationchromatography using a polystyrene standard in an art-recognized manner.

[0061] Non-limiting examples of film-forming resins suitable for use asthe resin (a) in anionic electrodepositable coating compositions includebase-solubilized, carboxylic acid group-containing polymers such as thereaction product or adduct of a drying oil or semi-drying fatty acidester with a dicarboxylic acid or anhydride; and the reaction product ofa fatty acid ester, unsaturated acid or anhydride and any additionalunsaturated modifying materials which are further reacted with polyol.Also suitable are the at least partially neutralized interpolymers ofhydroxy-alkyl esters of unsaturated carboxylic acids, unsaturatedcarboxylic acid and at least one other ethylenically unsaturatedmonomer. Still another suitable electrodepositable resin comprises analkyd-aminoplast vehicle, i.e., a vehicle containing an alkyd resin andan amine-aldehyde resin. Another suitable anionic electrodepositableresin composition comprises mixed esters of a resinous polyol. Thesecompositions are described in detail in U.S. Pat. No. 3,749,657 at col.9, lines 1 to 75 and col. 10, lines 1 to 13, all of which are hereinincorporated by reference. Other acid functional polymers can also beused such as phosphatized polyepoxide or phosphatized acrylic polymersas are well known to those skilled in the art. Additionally, suitablefor use as the resin (a) are those resins comprising one or more pendentcarbamate functional groups, for example, those described in U.S. Pat.No. 6,165,338.

[0062] In one particular embodiment of the present invention, the activehydrogen-containing ionic electrodepositable resin (a) is cationic andcapable of deposition on a cathode. Non-limiting examples of suchcationic film-forming resins include amine salt group-containing resinssuch as the acid-solubilized reaction products of polyepoxides andprimary or secondary amines such as those described in U.S. Pat. Nos.3,663,389; 3,984,299; 3,947,338; and 3,947,339. Usually, these aminesalt group-containing resins are used in combination with a blockedisocyanate curing agent as described in detail below. The isocyanate canbe fully blocked as described in the aforementioned U.S. Pat. No.3,984,299 or the isocyanate can be partially blocked and reacted withthe resin backbone such as described in U.S. Pat. No. 3,947,338. Also,one-component compositions as described in U.S. Pat. No. 4,134,866 andDE-OS No. 2,707,405 can be used in the electrodepositable coatingcompositions of the present invention as the resin (a). Besides theepoxy-amine reaction products discussed immediately above, the resin (a)can also be selected from cationic acrylic resins such as thosedescribed in U.S. Pat. Nos. 3,455,806 and 3,928,157.

[0063] Besides amine salt group-containing resins, quaternary ammoniumsalt group-containing resins can also be employed. Examples of theseresins include those which are formed from reacting an organicpolyepoxide with a tertiary amine salt. Such resins are described inU.S. Pat. Nos. 3,962,165; 3,975,346; and 4,001,101. Examples of othercationic resins are ternary sulfonium salt group-containing resins andquaternary phosphonium salt-group containing resins such as thosedescribed in U.S. Pat. Nos. 3,793,278 and 3,984,922, respectively. Also,film-forming resins which cure via transesterification such as describedin European Application No. 12463 can be used. Further, cationiccompositions prepared from Mannich bases such as described in U.S. Pat.No. 4,134,932 can be used.

[0064] In one embodiment of the present invention, the resin (a) cancomprise one or more positively charged resins which contain primaryand/or secondary amine groups. Such resins are described in U.S. Pat.Nos. 3,663,389; 3,947,339; and 4,116,900. In U.S. Pat. No. 3,947,339, apolyketimine derivative of a polyamine such as diethylenetriamine ortriethylenetetraamine is reacted with a polyepoxide. When the reactionproduct is neutralized with acid and dispersed in water, free primaryamine groups are generated. Also, equivalent products are formed whenpolyepoxide is reacted with excess polyamines such as diethylenetriamineand triethylenetetraamine and the excess polyamine vacuum stripped fromthe reaction mixture. Such products are described in U.S. Pat. Nos.3,663,389 and 4,116,900.

[0065] Mixtures of the above-described ionic resins also can be usedadvantageously. In one embodiment of the present invention, the resin(a) comprises a polymer having cationic salt groups and is selected froma polyepoxide-based polymer having primary, secondary and/or tertiaryamine groups (such as those described above) and an acrylic polymerhaving hydroxyl and/or amine functional groups.

[0066] As previously discussed, in one particular embodiment of thepresent invention, the resin (a) comprises cationic salt groups. In thisinstance, such cationic salt groups typically are formed by solubilizingthe resin with an inorganic or organic acid such as those conventionallyused in electrodepositable compositions. Suitable examples ofsolubilizing acids include, but are not limited to, sulfamic, acetic,lactic, and formic acids. Sulfamic and lactic acids are most commonlyemployed.

[0067] Also, as aforementioned, the covalently bonded halogen content ofthe resinous phase of the electrodepositable coating composition can bederived from halogen atoms covalently bonded to the resin (a). In suchinstances, the covalently bonded halogen content can be attributed to areactant used to form any of the film-forming ionic resins describedabove. For example, in the case of an anionic group-containing polymer,the resin may be the reaction product of a halogenated phenol, forexample a halogenated polyhydric phenol such as chlorinated orbrominated bisphenol A with an epoxy compound followed by reaction withphosphoric acid, or alternatively, an epoxy compound reacted with ahalogenated carboxylic acid, followed by reaction of any residual epoxygroups with phosphoric acid. The acid groups can then be solubilizedwith an amine. Likewise, in the case of a cationic salt group-containingpolymer, the resin may be the reaction product of diglycidyl ether ofBisphenol A with a halogenated phenol, follow by reaction of anyresidual epoxy groups with an amine. The reaction product then can besolubilized with an acid.

[0068] In one embodiment of the present invention, the covalently bondedhalogen content of the resin (a) can be derived from a halogenatedphenol, trichlorobutylene oxide, and mixtures thereof. In anotherembodiment of the present invention, the covalently bonded halogencontent of the resin (a) is derived from a halogenated polyhydricphenol, for example, chlorinated bisphenol A such astetrachlorobisphenol A, or a brominated bisphenol A such astetrabromobisphenol A. In a further embodiment of the present invention,the covalently bonded halogen content of the resin (a) can be derivedfrom a halogenated epoxy compound, for example the diglycidyl ether of ahalogenated bisphenol A.

[0069] The active hydrogen-containing ionic electrodepositable resin (a)described above can be present in the electrodepositable coatingcomposition of the present invention in amounts ranging from 5 to 90percent by weight, usually 10 to 80 percent by weight, often 10 to 70percent by weight, and typically 10 to 30 percent by weight based ontotal weight of the electrodepositable coating composition.

[0070] As mentioned above, the resinous phase of the electrodepositablecoating composition of the present invention further comprises (b) acuring agent adapted to react with the active hydrogens of the ionicelectrodepositable resin (a) described immediately above. Both blockedorganic polyisocyanate and aminoplast curing agents are suitable for usein the present invention, although blocked isocyanates typically areemployed for cathodic electrodeposition.

[0071] Aminoplast resins, which are common curing agents for anionicelectrodeposition, are the condensation products of amines or amideswith aldehydes. Examples of suitable amine or amides are melamine,benzoguanamine, urea and similar compounds. Generally, the aldehydeemployed is formaldehyde, although products can be made from otheraldehydes such as acetaldehyde and furfural. The condensation productscontain methylol groups or similar alkylol groups depending on theparticular aldehyde employed. Preferably, these methylol groups areetherified by reaction with an alcohol. Various alcohols employedinclude monohydric alcohols containing from 1 to 4 carbon atoms such asmethanol, ethanol, isopropanol, and n-butanol, with methanol beingpreferred. Aminoplast resins are commercially available from AmericanCyanamid Co. under the trademark CYMEL and from Monsanto Chemical Co.under the trademark RESIMENE.

[0072] The aminoplast curing agents typically are utilized inconjunction with the active hydrogen containing anionicelectrodepositable resin in amounts ranging from 1 to 90, often from 5percent to about 60 percent by weight, preferably from 20 percent to 40percent by weight, the percentages based on the total weight of theresin solids in the electrodepositable coating composition.

[0073] The curing agents commonly employed in cathodic electrodepositioncompositions are blocked polyisocyanates. The polyisocyanates can befully blocked as described in U.S. Pat. No. 3,984,299 column 1 lines 1to 68, column 2 and column 3 lines 1 to 15, or partially blocked andreacted with the polymer backbone as described in U.S. Pat. No.3,947,338 column 2 lines 65 to 68, column 3 and column 4 lines 1 to 30,which are incorporated by reference herein. By “blocked” is meant thatthe isocyanate groups have been reacted with a compound such that theresultant blocked isocyanate group is stable to active hydrogens atambient temperature but reactive with active hydrogens in the filmforming polymer at elevated temperatures usually between 90° C. and 200°C.

[0074] Suitable polyisocyanates include aromatic and aliphaticpolyisocyanates, including cycloaliphatic polyisocyanates andrepresentative examples include diphenylmethane-4,4′-diisocyanate (MDI),2,4- or 2,6-toluene diisocyanate (TDI), including mixtures thereof,p-phenylene diisocyanate, tetramethylene and hexamethylenediisocyanates, dicyclohexylmethane-4,4′-diisocyanate, isophoronediisocyanate, mixtures of phenylmethane-4,4′-diisocyanate andpolymethylene polyphenylisocyanate. Higher polyisocyanates such astriisocyanates can be used. An example would includetriphenylmethane-4,4′,4″-triisocyanate. Isocyanate prepolymers withpolyols such as neopentyl glycol and trimethylolpropane and withpolymeric polyols such as polycaprolactone diols and triols (NCO/OHequivalent ratio greater than 1) can also be used.

[0075] The polyisocyanate curing agents typically are utilized inconjunction with the active hydrogen containing cationicelectrodepositable resin (a) in amounts ranging from ranging from 1 to90 percent by weight, usually 1 to 80 percent by weight, often 1 to 70percent by weight, and typically 1 to 15 percent by weight based ontotal weight of the electrodepositable coating composition.

[0076] Also suitable are beta-hydroxy urethane curing agents such asthose described in U.S. Pat. Nos. 4,435,559 and 5,250,164. Suchbeta-hydroxy urethanes are formed from an isocyanate compound, forexample, any of those described immediately above, a 1,2-polyol and/or aconventional blocking such as monoalcohol. Also suitable are thesecondary amine blocked aliphatic and cycloaliphatic isocyanatesdescribed in U.S. Pat. Nos. 4,495,229 and 5,188,716.

[0077] As previously discussed, in one embodiment of the presentinvention, the curing agent (b) can have a covalently bonded halogencontent of up to 60 weight percent, often from 1 to 50 weight percent,often from 2 to 80 weight percent, usually from 5 to 25 weight percent,and can be from 10 to 20 weigh percent based on weight of total resinsolids present in the curing agent (b). In such instances, thecovalently bonded halogen content present in the curing agent (b) can bederived from for example, a halogen containing blocked isocyanate can beprepared by at least partially blocking 4-chloro-6-methyl-1,3-phenylenediisocyanate with a suitable blocking agent such as 2-butoxy ethanol, Ifpartially blocked, any residual isocyanate groups can be reacted with apolyol such as trimethol propane thereby increasing the molecular weightof the curing agent.

[0078] In a further embodiment of the present invention, the covalentlybonded halogen content present in the resinous phase of theelectrodepositable coating composition can be derived from a component(c) which is different from and present in addition to the resin (a) andthe curing agent (b). In such instances, the component (c) typically isa covalently bonded halogen-containing compound selected from the groupconsisting of halogenated polyolefin, halogenated phosphate ester,halogenated phenol such as any of the halogenated phenols describedabove.

[0079] As aforementioned, the covalently bonded halogen content presentin the resinous phase of the electrodepositable coating composition canbe derived from the resin (a), the curing agent (b), the component (c),or any combination of the foregoing, provided that the covalently bondedhalogen content is sufficient to ensure that the resultantelectrodeposition coating when electrophoretically applied and curedpasses flame resistance testing in accordance with IPC-TM-650 aspreviously discussed. The covalently bonded halogen content of theresinous phase of the electrodepositable coating composition also shouldbe present in an amount insufficient to adversely affect theelectrodeposition process and/or the resulting dielectric coatingproperties.

[0080] In one embodiment of the present invention, theelectrodepositable coating compostion can further comprise a rheologymodifier which can assist in the deposition of a smooth and uniformthickness of the dielectric coating on the surface of the hole or viawalls as well as the edges at the via openings. Any of a variety of therheology modifiers well-known in the coatings art can be employed forthis purpose.

[0081] One suitable rheology modifier comprises a cationic microgeldispersion prepared by dispersing in aqueous medium a mixture of acationic polyepoxide-amine reaction product which contains amine groups,typically primary amine groups, secondary amine groups and mixturesthereof, and a polyepoxide crosslinking agent, and heating the mixtureto a temperature sufficient to crosslink the mixture, thus forming acationic microgel dispersion. Such cationic microgel dispersions andtheir preparation are described in detail in U.S. Pat. No. 5,096,556 atcolumn 1, line 66 to column 5, line 13, incorporated by referenceherein. Other suitable rheology modifiers include the cationic microgeldispersion having a shell-core morphology described in detail in EP 0272 500 B1. This microgel is prepared by emulsification in aqueousmedium of a cationic film-forming resin and a thermosetting crosslinkingagent, and heating the resultant emulsion to a temperature sufficient tocrosslink the two components.

[0082] The cationic microgel is present in the electrodepositablecoating composition in an amount sufficient to effect adequate rheologycontrol and hole edge coverage, but insufficient to adversely affectflow of the electrodepositable composition upon application or surfaceroughness of the cured coating. For example, the cationic microgelsdescribed immediately above can be present in the resinous phase of theelectrodepositable coating composition in an amount ranging from 0.1 to30 weight percent, typically from 1 to 10 weight percent based on weightof total resin solids present in the resinous phase.

[0083] The electrodepositable coating composition is in the form of anaqueous dispersion. The term “dispersion” is believed to be a two-phasetransparent, translucent or opaque resinous system in which the resin isin the dispersed phase and the water is in the continuous phase. Theaverage particle size of the resinous phase is generally less than 1.0micron, usually less than 0.5 micron, and typically less than 0.15micron.

[0084] The concentration of the resinous phase in the aqueous medium isat least 1, and usually from 2 to 60 percent by weight based on totalweight of the aqueous dispersion. When the compositions of the presentinvention are in the form of resin concentrates, they generally have aresin solids content of 20 to 60 percent by weight based on weight ofthe aqueous dispersion.

[0085] Electrodepositable coating compositions of the inventiontypically are supplied as two components: (1) a clear resin feed, whichincludes, generally, the active hydrogen-containing ionicelectrodepositable resin, i.e., the main film-forming polymer, thecuring agent, and any additional water-dispersible, non-pigmentedcomponents; and (2) a pigment paste, which, generally, includes one ormore pigments, a water-dispersible grind resin which can be the same ordifferent from the main-film forming polymer, and, optionally, additivessuch as catalysts, and wetting or dispersing aids. Electrodepositablecoating components (1) and (2) are dispersed in an aqueous medium whichcomprises water and, usually, coalescing solvents to form anelectrodeposition bath. Alternatively, the electrodepositablecomposition of the present invention can be supplied as a one-componentcomposition. In a particular embodiment of the present invention, theelectrodepositable coating composition can be supplied as asubstantially pigment-free, one-component composition.

[0086] It should be appreciated that there are various methods by whichthe component (c), when employed, can be incorporated into theelectrodepositable coating composition in the form of anelectrodeposition bath. The component (c) can be incorporated “neat”,that is, the component (c) or an aqueous solution thereof can be addeddirectly to the dispersed electrodeposition composition components (1)and (2), or if applicable, to the dispersed one-componentelectrodeposition composition. Alternatively, the component (c) can beadmixed with or dispersed in the clear resin feed (or any of theindividual clear resin feed components, for example the film-formingresin or the curing agent) prior to dispersing components (1) and (2) inthe aqueous medium. Further, the component (c) can be admixed with ordispersed in the pigment paste, or any of the individual pigment pastecomponents, for example, the pigment grind resin prior to dispersingcomponents (1) and (2) in the aqueous medium. Finally the component (c)can be added on-line directly to the electrodeposition bath.

[0087] The electrodepositable coating composition in the form of anelectrodeposition bath typically has a resin solids content within therange of 5 to 25 percent by weight based on total weight of theelectrodeposition bath.

[0088] As aforementioned, besides water, the aqueous medium may containa coalescing solvent. Useful coalescing solvents include hydrocarbons,alcohols, esters, ethers and ketones. Usual coalescing solvents includealcohols, polyols and ketones. Specific coalescing solvents includeisopropanol, butanol, 2-ethylhexanol, isophorone, 2-methoxypentanone,ethylene and propylene glycol and glycol ethers such as monoethyl,monobutyl and monohexyl ethers of ethylene glycol. The amount ofcoalescing solvent is generally between about 0.01 and 25 percent andwhen used, preferably from about 0.05 to about 5 percent by weight basedon total weight of the aqueous medium.

[0089] Although typically substantially free of pigment, if desired, apigment composition and/or various additives such as surfactants,wetting agents or catalyst can be included in the dispersion. Thepigment composition may be of the conventional type comprising pigments,for example, iron oxides, strontium chromate, carbon black, titaniumdioxide, talc, barium sulfate, as well as color-imparting pigments wellknown in the art. The electrodeposition bath can be, if desired,essentially free of chrome- and/or lead-containing pigments.

[0090] The pigment content of the dispersion is usually expressed as apigment-to-resin ratio. In the practice of the invention, when pigmentis employed, the pigment-to-resin ratio is usually within the range ofabout 0.02 to 1:1. The other additives mentioned above are usually inthe dispersion in amounts ranging from 0.01 to 10 percent by weightbased on weight of resin solids.

[0091] Illustrating the invention are the following examples which arenot to be considered as limiting the invention to their details. Unlessotherwise indicated, all parts and percentages in the followingexamples, as well as throughout the specification, are by weight.

EXAMPLES

[0092] The following describes the preparation of a circuitizedsubstrate using the process of the present invention. Example Adescribes the preparation of a resinous binder comprised of atetrabromobisphenol A which is used in the electrodepositable coatingcomposition of Example 1. The electrodepositable coating composition ofExample 1 in the form of an electrodeposition bath was used to provide aconformal dielectric coating on a perforate substrate, which wassubsequently metallized, photoimaged, developed and stripped asdescribed below.

Example A

[0093] This example describes the preparation of a cationic resinousbinder used in the electrodepositable coating composition of thefollowing Example 1. The resinous binder was prepared as described belowfrom the following ingredients. All values listed represent parts byweight in grams. Ingredients Example A Crosslinker¹ 1882 Diethyleneglycol monobutyl 108.78 ether formal EPON ® 828² 755.30Tetrabromobisphenol A 694.90 TETRONIC 150R1³ 0.33 Diethanolamine 51.55Aminopropyl diethanolamine 113.2 Distillate removed (67.66) Sulfamicacid 45.17 Deionized water 2714 Lactic acid⁴ 1.70 Resin intermediate⁵244.7 Gum rosin⁵ 27.49 Deionized water 2875 ¹Polyisocyanate curing agentprepared from the following ingredients: Parts by Weight Ingredients(grams) Ethanol 92.0 Propylene glycol 456.0 Polyol^(a) 739.5Methylisobutyl ketone 476.5 Diethylene glycol monobutyl 92.8 etherformal^(b) DESMODUR LS2096^(c) 1320.0 Methylisobutyl ketone 76.50^(a)Bisphenol A/ethylene oxide adduct available from BASF Corporation asMACOL 98B. ^(b)Available from BASF Corporation as MAZON 1651.^(c)Isocyanate available from Bayer Corporation.  The first fiveingredients were charged to a suitably equipped reaction vessel underagitation. As the temperature reached about 25° C., the addition ofDESMODUR LS2096 was begun. Temperature was increased to 105° C. at whichtime the last addition of methylisobutyl ketone was made. Temperaturewas held at 100° C. as the reaction was monitored for the disappearanceof NCO by infrared spectroscopy at which time, the temperature wasreduced to 80° C. ²Diglycidyl ether of bisphenol A available from ShellOil and Chemical Company. ³Surfactant available from BASF Corporation.⁴88% aqueous solution. ⁵Cationic resin prepared from the followingingredients. Parts by Weight Ingredients (grams) MAZEEN 355 70^(a)603.34 Acetic acid 5.99 Dibutyltindilaurate 0.66 Toluene diisocyanate87.17 Sulfamic acid 38.79 Deionized water 1289.89 ^(a)Aminediolavailable from BASF Corporation  The first two ingredients were chargedto a suitably equipped reaction vessel and agitated for 10 minutes atwhich time dibutyl- tindilaurate was added. The toluene diisocyanate wasadded slowly as the reaction was permitted to exotherm to a temperatureof 100° C. and held at that temperature until the disappearance of allNCO as monitored by infrared spectroscopy. The resin thus prepared wassolubilized with the addition of sulfamic acid and de- ionized waterunder agitation. The final dispersion had a measured resin solidscontent of 26 percent by weight.

[0094] The crosslinker was added to a suitably equipped reaction vessel.The next four ingredients were added to the reaction vessel under mildagitation and the reaction mixture was heated to a temperature of 75° C.at which time the diethanolamine was added. The reaction mixture washeld at that temperature for a period of 30 minutes. The aminopropyldiethanolamine was then added and the reaction mixture was permitted toexotherm to 132° C. and held at that temperature for a period of 2hours. Distillate was removed. For solubilization, the reaction productwas added under mild agitation to an admixture of the sulfamic acid,deionized water, lactic acid solution and cationic resin intermediate.The gum rosin solution was then added to the solubized resin, followedby deionized water in two sequential additions. Excess water and solventwere removed by stripping under vacuum at a temperature of 60°-65° C.The final reaction product had a measured resin solids content ofapproximately 40 percent by weight.

Example 1

[0095] The following example describes the preparation of anelectrodepositable coating composition of the present invention in theform of an electrodeposition bath comprising the cationic resinousbinder of Example A above. The electrodepositable coating compositionwas prepared as described below from the following ingredients. Allvalues listed represent parts by weight in grams. Ingredients Example 1Resinous binder of Example A 704.9 Hexyl cellosolve 28.5 E6278¹ 13.2Deionized water 3053.4

[0096] The ingredients listed above were combined and mixed with mildagitation. The composition was ultrafiltered 50% and reconstituted withdeionized water.

[0097] Preparation of a Circuitized Substrate:

[0098] A single layer of INVAR metal perforate (50 micron thickness)containing 200 micron diameter holes positioned 500 microns apart(center-to-center) in a square grid array (supplied by Buckbee-Mears, adivision of BMC Industries, Inc.) was cleaned and micro-etched to removeundesirable dirt, oils and oxides. The pre-cleaned perforate substratethen was electroplated to provide a layer of copper metal having athickness of 9 microns.

[0099] The electrodepositable coating composition of Example 1 above waselectrophoretically applied to the electroplated substrate in anelectrodeposition bath at a temperature of 105° F. (41° C.) at 90 Voltsfor 2 minutes. The electrocoated substrate was rinsed with deionizedwater and air dried such that all holes of the perforate substrate werefree of water. The electrocoated substrate was heated to a temperatureof 350° F. (177° C.) for 30 minutes to cure the electrodepositablecoating, thereby providing a cured dielectric film thickness of 20microns.

[0100] The electrocoated substrate was then metallized. The substratewas heated to a temperature of 50° C. for a period of 30 minutes todrive off any moisture that may have been absorbed during handling. Thesubstrate thus dried was immediately introduced into a vacuum chamberfor plasma treatment with argon ions to activate the coating surface.The substrate surface was then sputter coated with 200 angstroms ofnickel followed by sputter coating with 3000 angstroms of copper. Themetal layer thus formed was electroplated with an additional 9 micronsof copper.

[0101] The metallized substrate was cleaned and micro-etched to removeany dirt, oils or oxides from the metal surface, thenelectrophoretically coated with ELECTROIMAGE® PLUS photoresist(available from PPG Industries, Inc.) at a temperature of 84° F. (29°C.) at 80 Volts for 90 seconds. The coated substrate then was rinsedwith deionized water and heated to a temperature of 250° F. (120° C.)for 6 minutes to drive off any residual solvents and/or water. Aphotoresist coating having a dry film thickness of 5 microns wasobtained. The coated substrate then was exposed to an ultraviolet lightsource through a phototool on each side, and developed withELECTROIMAGE® Developer EID-523, photoresist developing solution(available from PPG Industries, Inc.) to expose copper in pre-selectedareas. The exposed copper areas were etched with a cupric chloride acidetchant and the remaining photoresist stripped with ELECTROIMAGE®stripper EID-568 photoresist stripping solution (available from PPGIndustries, Inc.), thereby providing a circuitized substrate.

[0102] It will be appreciated by those skilled in the art that changescould be made to the embodiments described above without departing fromthe broad inventive concept thereof. It is understood, therefore, thatthis invention is not limited to the particular embodiments disclosed,but it is intended to cover modifications which are within the spiritand scope of the invention, as defined by the appended claims.

Therefore we claim:
 1. A process for forming metallized vias in asubstrate comprising the following steps: (I) electrophoreticallyapplying to an electroconductive substrate an electrodepositable coatingcomposition onto all exposed surfaces of the substrate to form aconformal dielectric coating thereon, said electrodepositable coatingcomposition comprising a resinous phase dispersed in an aqueous phase,said resinous phase comprising: (a) an ungelled, activehydrogen-containing, ionic group containing resin, and (b) a curingagent reactive with the active hydrogens of the resin (a), said resinousphase having a covalently bonded halogen content of at least 1 percentby weight based on total weight of resin solids present in said resinousphase, (II) ablating a surface of the conformal dielectric coating in apredetermined pattern to expose one or more section of the substrate;(III) applying a layer of metal to all surfaces to form metallized viasin the substrate.
 2. The process of claim 1, wherein the vias extend tothe substrate surface.
 3. The process of claim 1, wherein the viasextend through the substrate.
 4. The process of claim 1, wherein thesubstrate comprises a metal selected from perforate copper foil, aniron-nickel alloy and combinations thereof.
 5. The process of claim 1,wherein the conformal dielectric coating is heated at a temperature andfor a time sufficient to cure the dielectric coating prior to orsubsequent to step (II).
 6. The process of claim 5, wherein the cureddielectric coating passes flame resistance testing in accordance withIPC-TM-350 and has a dielectric constant of no more than 3.50.
 7. Theprocess of claim 6, wherein the cured dielectric coating has adielectric constant of no more than 3.30.
 8. The process of claim 6,wherein the cured dielectric coating has a dielectric constant of nomore than 3.00.
 9. The process of claim 6, where in the cured dielectriccoating has a dielectric loss factor of no more than 0.01.
 10. Theprocess of claim 1, wherein the resinous phase of the electrodepositablecoating composition has a covalently bonded halogen content ranging from1 to 50 weight percent based on weight of total resin solids present inthe resinous phase.
 11. The process of claim 1, further comprising step(IV) applying a resinous photosensitive layer to the metal layer of step(III).
 12. The process of claim 5, wherein the cured dielectric coatinghas a film thickness of no more than 25 microns.
 13. A process forfabricating a circuit assembly comprising the following steps: (I)providing an electroconductive core (II) applying electrophoretically anelectrodoepositable coating composition onto all exposed surfaces of thecore to form a conformal dielectric coating thereon, saidelectrodepositable coating composition comprising a resinous phasedispersed in an aqueous phase, said resinous phase comprising: (a) anungelled, active hydrogen-containing, ionic salt group-containing resin;and (b) a curing agent reactive with the active hydrogens of the resin(a), said resinous phase having a covalently bonded halogen content ofat least 1 percent by weight based on total weight of resin solidspresent in said resinous phase; (III) ablating a surface of theconformal dielectric coating in a predetermined pattern to expose asection of the core; (IV) applying a layer of metal to all surfaces toform metallized vias in the core; and (V) applying a resinousphotosensitive layer to the metal layer.
 14. The process of claim 13,wherein the core is a metal core selected from perforate copper foil, aniron-nickel alloy, and combinations thereof.
 15. The process of claim14, wherein the metal core comprises a perforate copper foil.
 16. Theprocess of claim 14, wherein the metal core comprises an iron-nickelalloy.
 17. The process of claim 13, wherein the resin (a) comprises apolymer derived from at least one of a polyepoxide polymer and anacrylic polymer.
 18. The process of claim 17, wherein the resin (a)comprises cationic salt groups selected from amine salt groups and/oronium salt groups.
 19. The process of claim 13, wherein the resin (a)has a covalently bonded halogen content ranging from 1 to 50 percent byweight based on total weight of resin solids present in the resin (a).20. The process of claim 19, wherein the covalently bonded halogencontent of the resin (a) is derived from a halogenated polyhydric phenolselected from at least one of chlorinated polyhydric phenol andbrominated polyhydric phenol.
 21. The process of claim 19, wherein thecovalently bonded halogen content present in the resin (a) is derivedfrom tetrabromobisphenol A.
 22. The process of claim 13, wherein thecuring agent (b) is selected from at least one of a blockedpolyisocyanate and an aminoplast resin.
 23. The process of claim 13,wherein the curing agent (b) has a covalently bonded halogen contentranging from 1 to 50 percent by weight based on total weight of resinsolids present in the curing agent (b)
 24. The process of claim 13,wherein the covalently bonded halogen content of the resinous phase ofthe electrodepositable coating composition is derived at least in partfrom a component (c) which is different from and present in addition tothe resin (a) and the curing agent (b).
 25. The process of claim 24,wherein component (c) comprises a covalently bonded halogen-containingcompound selected from the group consisting of halogenated polyolefin,halogenated phosphate ester, halogenated phenol, and mixtures thereof.26. The process of claim 13, wherein the electrodepositable coatingcomposition further comprises a rheology modifier.
 27. The process ofclaim 26, wherein the rheology modifier comprises a cationic microgeldispersion prepared by dispersing in aqueous medium a mixture of acationic polyepoxide-amine reaction product which contains amine groupsselected from the group consisting of primary amne groups, secondaryamine groups and mixtures thereof and a polyepoxide crosslinking agent,and heating said mixture to a temperature sufficient to crosslink themixture to form said cationic microgel dispersion.
 28. The process ofclaim 13, wherein prior to step (III), said conformal coating is heatedto a temperature and for a time sufficient to cure said coating.
 29. Theprocess of claim 28, wherein said cured conformal coating has adielectric constant of less than or equal to 3.50.
 30. The process ofclaim 28, wherein said cured conformal coating passes flame resistancetesting in accordance with IPC-TM-650.
 31. The process of claim 29,wherein said cured conformal coating has a dry film thickness of lessthan or equal to 25 microns.
 32. A process for fabricating a circuitassembly comprising the following steps: (I) providing anelectroconductive core; (II) providing a photoresist at predeterminedlocations on the surface of the core; (III) applying electrophoreticallyan electrodepositable coating composition over the core of step (II),wherein said coating composition is deposited electrophoretically overall surfaces of the core except at the locations having the photoresistthereon, said electrodepositable coating composition comprising aresinous phase dispersed in an aqueous phase, said resinous phasecomprising: (a) an ungelled, active hydrogen-containing, ionic saltgroup-containing resin; and (b) a curing agent reactive with the activehydrogens of the resin (a), said resinous phase having a covalentlybonded halogen content of at least 1 percent by weight based on totalweight of resin solids present in said resinous phase; (IV) curing theelectrophoretically applied coating composition to form a curedconformal dielectric layer over all surfaces of the core except at thelocations having the photoresist thereon; (V) removing said photoresistto form a circuit assembly having vias extending to said core at thelocations previously covered with said resist; and (VI) optionally,applying a layer of metal to all surfaces of the circuit assembly ofstep (V) to form metallized vias extending to said core.
 33. The processof claim 32, wherein prior to providing the photoresist in step (II) atpredetermined locations on the surface of the core, a layer of coppermetal is applied to the core.
 34. The process of claim 32, wherein thecore comprises a metal core selected from perforate copper foil,iron-nickel alloy, and combinations thereof.
 35. The process of claim34, wherein the metal core comprises an iron-nickel alloy.
 36. Theprocess of claim 34, wherein the metal core comprises perforate copperfoil.
 37. The process of claim 32, wherein the resin (a) comprises apolymer derived from at least one of a polyepoxide polymer and anacrylic polymer.
 38. The process of claim 37, wherein the resin (a)comprises cationic salt groups selected from amine salt groups and/oronium salt groups.
 39. The process of claim 32, wherein the resin (a)has a covalently bonded halogen content ranging from 1 to 50 percent byweight based on total weight of resin solids present in the resin (a).40. The process of claim 39, wherein the covalently bonded halogencontent of the resin (a) is derived from a halogenated polyhydric phenolselected from at least one of chlorinated polyhydric phenol andbrominated polyhydric phenol.
 41. The process of claim 40, wherein thecovalently bonded halogen content of the resin (a) is derived fromtetrabromobisphenol A.
 42. The process of claim 32, wherein the curingagent (b) comprises a compound selected from at least on of a blockedpolyisocyanate and an aminoplast resin.
 43. The process of claim 32,wherein the curing agent (b) has a covalently bonded halogen contentranging from 1 to 50 percent by weight based on total weight of resinsolids present in the curing agent (b).
 44. The process of claim 32,wherein the covalently bonded halogen content of the resinous phase isderived at least in part from a component (c) which is different fromand present in addition to the resin (a) and the curing agent (b). 45.The process of claim 44, wherein component (c) comprises one or morecovalently bonded halogen-containing compounds selected from the groupconsisting of halogenated polyolefin, halogenated phosphate ester,halogenated phenol, and mixtures thereof.
 46. The process of claim 32,wherein said cured conformal dielectric coating has a dielectricconstant of less than or equal to 3.50.
 47. The process of claim 46,wherein said cured conformal dielectric coating has a dielectricconstant of less than or equal to 3.30.
 48. The process of claim 46,wherein said cured conformal coating has a film thickness of less thanor equal to 25 microns.
 49. A substrate formed by the process ofclaim
 1. 50. A circuit assembly formed by the process of claim
 13. 51. Acircuit assembly formed by the process of claim 32.